The present invention relates to apparatus for analyzing the surface of a sample of interest and, more particularly, to novel scanning micro-probe imaging subsystems for analyzing the properties of a surface of a sample of interest and obtaining a spectral analysis thereof by utilizing a modulated radiation beam to measure the spectral response of a region which is smaller than the wavelength of the illuminating radiation spectrum.
Several detection methods have been proposed for use in scanning micro-probe imaging systems. The original micro-probe method utilized tunneling current as a measure of the proximity of the probe tip to one minute portion the surface of interest. Other methods have utilized measurements of the probe strain to map each small point on the adjacent surface. In none of the methods is a spectroscopic analysis of the surface characterization provided. It is known to extract spectroscopic information from a large (e.g. approximate 1 square cm.) area of a surface with a relatively new technology known as photoacoustic spectroscopy, which exploits the fact that an object heats up upon absorbing radiation and, if the radiation is pulsed, the heat absorbed by the object will be released in a pulsed fashion to the object's environment. In photoacoustic spectrometers, modulated surface heat transferred to an adjacent gas, typically air, is sufficient to cause detectable sound waves, provided by physical displacement of the gas at the modulation frequency rate. Thus, if the incident light wavelength is changed during data acquisition, the absorption spectrum from the surface point can be detected, although a major problem of photoacoustic spectroscopy is that its sensitivity is not great and does not allow photoacoustic spectroscopy to be well suited for sample analysis, even though this form of spectroscopy has proven extremely useful for spectroscopic analysis of non-transparent objects and especially object surfaces which would otherwise be impossible to analyze, provided a sample is of sufficiently large size. Accordingly, a scanning micro-probe spectral analysis imaging system utilizing a modulated radiation beam for detecting surface property changes to provide sample of surface information, and particularly spectroscopic information, is highly desirable.